1. Field of the Invention
The present invention relates generally to cementing subterranean zones penetrated by well bores, and more particularly, to such methods whereby high strength foamed well cement compositions which are crack and shatter resistant are utilized.
2. Description of the Prior Art
Light weight foamed hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, foamed hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing where light weight cement compositions are required to prevent fracturing of subterranean zones, light weight foamed hydraulic cement compositions are pumped into the annular space between the walls of a well bore penetrating the zones and the exterior surfaces of a pipe string disposed therein. The light weight foamed cement composition is permitted to set in the annular space thereby forming an annular sheath of hard substantially impermeable foamed cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Multi-lateral wells have recently been developed which include vertical or deviated principal well bores having one or more ancillary laterally extending well bores connected thereto. Drilling and completion equipment is available which allows multiple laterals to be drilled from a principal cased and cemented well bore. Each of the lateral well bores can include a liner cemented therein which is tied into the principal well bore. The lateral well bores can be drilled into predetermined producing formations or zones at any time in the productive life cycle of the well.
In both conventional wells having single well bores and multi-lateral wells having several well bores, the light weight foamed cement compositions utilized for cementing casings or liners in the well bores must have sufficient strength, resiliency, ductility and toughness to resist cracking or shattering as a result of pipe movements, impacts and/or shocks subsequently generated by drilling and other well operations such as perforating. Set foamed cement in wells, and particularly, a set foamed cement sheath in the annulus between a pipe string and the walls of a well bore often fails by cracking or shattering during drilling and/or perforating operations. When the set cement cracks or shatters, rubble is often produced and the seal between the pipe and the walls of the well bore is lost.
Various types of fibers have been utilized in construction cement compositions heretofore. The fibers have been formed of glass, steel, graphite, polyesters, polyamides and polyolefins. Polyolefin fibers have generally been the most preferred in that they are readily available, are low in cost and have high resistance to corrosion and degradation. Fibrillated net-shaped polyolefin fibers have been found to be particularly suitable for use in construction cement compositions because they resist being pulled out of the set cement. The fibers also function to control shrinkage cracking in the early stages of the cement setting process, and after setting, the fibers provide resiliency, ductility and toughness to the cement composition whereby it resists cracking or shattering. When cracking or shattering does occur, the fibers hold the cracked or shattered set cement together and prevent the formation of rubble.
While fibrillated polyolefin net-shaped fibers have been included in well cement compositions heretofore, problems have been encountered because the fibers have been hydrophobic and are difficult to dry blend with cement. Also, the fibers agglomerate in the dry cement when it is conveyed causing plugging to occur, and when the cement and hydrophobic fibers are combined with mixing water, the hydrophobic fibers form mats which prevent their dispersion into and throughout the cement slurry. The lack of dispersion of the fibers in the cement slurry make it difficult to pump and foam.
Thus, there are needs for improved foamed well cement compositions and methods wherein the cement compositions contain fibrillated fibers which can be easily mixed and conveyed with dry cement and subsequently dispersed in the aqueous cement slurry formed prior to pumping and foaming the slurry.